Experimental Study On Rock Fracture Expansion Law Under Ultrasonic Excitation

At present,underground rock engineering construction is often faced with hard rock fragmentation.Traditional rock fragmentation methods show problems of low efficiency,high consumption and high cost.It is extremely important and urgent to improve the efficiency of hard rock fragmentation and innovate rock fragmentation methods.Some scholars have gradually carried out the research of high-efficiency rock breaking technology by ultrasonic wave,but it is still in the initial stage,lacking systematic theory and related experimental data.Based on theoretical analysis and laboratory experiments,this paper discusses the mechanism of rock breaking by ultrasonic vibration,and analyses the law of rock crack propagation under ultrasonic vibration,in order to provide experimental data for the formation of a complete and mature technology of high-efficiency rock breaking by ultrasonic.The main results of this paper are as follows:(1)The dynamic model of rock vibration excited by ultrasonic wave is established.The conditions of crack propagation,the length and speed of crack propagation and the effective failure depth of rock under ultrasonic vibration are analyzed.The fatigue failure mechanism of rock is discussed.It is considered that with the increase of elastic modulus of rock,the vibration limit offset of crack initiation increases,and the difficulty of rock fragmentation under ultrasonic excitation increases.(2)The apparent crack growth of specimens with different lithology and size after ultrasonic excitation for different time is analyzed.The results show: a)With the increase of excitation time,the apparent crack expands gradually from the edge of the specimen to the middle,and the maximum longitudinal extension depth increases gradually,but the increment of extension depth and the average expansion speed are different in different unit time intervals.b)Under the same conditions,the smaller the specimen size is,the higher the crack growth degree is,the larger the additional static pressure is,the greater the extension depth and average velocity are,and the smaller the excitation area is,the larger the average expansion speed of the longest apparent longitudinal crack is than that of the large excitation area.Because of the higher strength and being composed of high hardness quartz and feldspar,the time required to produce apparent cracks of granite is longer than that of red sandstone,limestone and marble in turn.c)Rock fracture propagation under ultrasonic vibration can be roughly divided into four stages: no obvious crack,crack initiation,smooth and rapid expansion.(3)The internal crack propagation of the same size and different lithologic specimens stimulated by ultrasonic wave for different time is analyzed.The results show: a)The trend of maximum longitudinal propagation depth and average propagation velocity of internal fissures with time and is basically the same witn the surface fissures,but both of them are smaller than surface fissures.b)From the vibration excitation contact surface to the bottom of the specimen,the internal cracks gradually extend to the edge of the specimen in the longitudinal direction.c)The closer to the excitation vibration contact surface,the more cracks developed in the internal expansion.(4)The micro-crack propagations of different lithologic specimens with the same size under the same excitation condition after different time of ultrasonic excitation vibration are analyzed.The results show: a)Because rocks are composed of mineral particles and cements,the micro-cracks of rocks in the early stage of stimulation mainly extend through intergranular cracks and intergranular cracks,and in the later stage,because of the different dielectric constants of various mineral grains,the transgranular cracks expand when the grain temperature rises due to friction.b)Tensile fracture is the main type of micro-crack extension shared by red sandstone,marble,granite and limestone.c)The transgranular crack extension surfaces of red sandstone and marble are smooth,while those of granite and limestone are rough,and the inflection points of transgranular crack extension are many and irregular.d)There are more short and small cracks on the surface of micro-grains of granite and limestone.There are totally 80 pictures,14 tables and 98 references in this paper.